Back to the OP, its very difficult to give any sort of tip to keep the van cool. As Damian quite rightly says, storage sites dont provide an EHU so anything electrically operated is going to drain the battery PDQ. Leaving windows open, even on the security latch may very well invalidate the insurance so I would be wary about that one. Personally I have never seen a window "go" due to the heat build up from blinds, but there is a lot of thought about the subject and I guess it holds that there is no smoke without fire. Keeping the curtains closed, or almost closed would help, but I suspect that the results would be minimal.
IDK that youcan get solar powered fans for cars, which would probably work in a caravan, but I think that with all the doors and windows shut, all that is going to do is circulate the hot air. I wonder is a solar powered fan could be hooked up direct to the roof solar panel if indeed the OP has one?